1. Field of the Invention
The present invention relates to a semiconductor device and a method for forming the same, and in particular, to a phase change memory device and a method for forming the same.
2. Description of the Related Art
Recently, a phase change memory device has been suggested as a new semiconductor device. The phase change memory device has a nonvolatile property of maintaining stored data when power supply is stopped. A unit cell of the phase change memory device uses a phase change material as a data storing medium. The phase change material has two stable states (amorphous state and crystalline state) by a provided heat. A generally-known phase change material is a compound of Ge, Sb and Te, commonly referred to as GST material (Ge—Sb—Te).
When the GST is heated for a short time at a temperature close to a melting point and is then sharply cooled, the GST has an amorphous state. If the GST is heated for a long time at a crystallization temperature lower than the melting point and slowly cooled, the GST has a crystalline state. Here, the amorphous GST has a higher specific resistance than the crystalline GST. Therefore, whether the information stored in the phase change memory cell is logical ‘1’ or ‘0’ can be decided by sensing an amount of current flowing through the phase change material.
Joule heat is used as the heat supplied to the phase change material. That is, when the current is supplied to an electrode connected to the phase change material, the Joule heat is generated from the electrode and supplied to the phase change material. The temperature of the heat supplied to the phase change material is dependent upon the amount of the supplied current.
FIG. 1 is a cross-sectional diagram illustrating a data storing medium of a conventional phase change memory cell.
Referring to FIG. 1, an interlayer insulation film 2 is disposed on a semiconductor substrate. A lower electrode 4 fills a contact hole 3 exposing a predetermined region of the semiconductor substrate 1 through the interlayer insulation film 2. The lower electrode 4 is formed in a contact plug shape. A GST film pattern 5 and an upper electrode 6 are sequentially stacked on the interlayer insulation film 2. The GST film pattern 5 contacts the top surface of the lower electrode 4. The GST film pattern 5 is formed in a crystalline state.
To program or erase the GST film pattern 5, a current is supplied to the lower electrode 4. The Joule heat is generated from the lower electrode 4, and supplied to the GST film pattern 5 through the contact surface between the GST film pattern 5 and the lower electrode 4. Depending on the amount of the supplied current and the duration of application of the current, a program region 7 of the GST film pattern 5 has the amorphous or crystalline state.
The program region 7 is formed to sufficiently cover the lower electrode 5. There is thus a distinct difference in the amount of the current flowing through the GST film pattern 5 according to the state (amorphous or crystalline state) of the GST film pattern 5. Since the generated heat is isotropically supplied to the GST film pattern 5, the program region 7 is formed in a hemispherical shape.
With the high integration and low power consumption tendency of the semiconductor device, research has been made to reduce the amount of the operation current (program or erase operation current) of the phase change memory device. Specifically a large amount of operation current is required to generate a high temperature for changing the state of the GST film pattern 5. Accordingly, a channel width of a MOS transistor for controlling the operation current increases. There is thus a limit in the ability to reduce the size of the phase change memory device.
One method for reducing the operation current of the phase change memory device is to reduce the width of the contact hole 3. As the width of the contact hole 3 decreases, the sectional width of the lower electrode 4 decreases, and the resistance of the lower electrode 4 increases. Even if the operation current decreases, the high temperature Joule heat can be generated. However, the contact hole 3 is formed by a patterning process including a photolithography process. As a result, there is a limit to reduction in the width of the contact hole 3 due to the limit of the photolithography process.